Broadband linear slot antenna with impedance matching network



Fb. 7, 1967 w, BACON ET AL 3,303,505

BROADBAND LINEAR SLOT ANTENNA WITH IMPEDANCE MATCHING NETWORK FiledSept. 24, 1957 3 Sheets-Sheet 1 Feb. 7, 1967 BACON ET AL 3,303,505

BROADBAND LINEAR SLOT ANTENNA WITH IMPEDANCE MATCHING NETWORK FiledSept. 24, 195'? s Sheets-Sheet s United States Patent 3,303 505BROADBAND LINEAR S LOT ANTENNA WITH IMPEDANCE MATCHING NETWORK WilfredH. Bacon, Hugh Gordon Byers, and Max Katchky, all of Scarboro, Ontario,Canada, assignors to Canadian Arsenals Limited, Ottawa, Ontario, Canada,

a corporation of Canada Filed Sept. 24, 1957, Ser. No. 685,996 4 Claims.(Cl. 343-771) This invention relates to linear array antennae for use atmicrowave frequencies and more particularly to squintfree broadsidelinear array antennae.

A linear array is defined as one in which the beam pattern is formed bya series of radiating elements disposed along a common linear axis, theshape and direction of this beam pattern being controlled by therelative amplitude and phase of the currents in the elements of thearray; the spacing of the elements along this axis; and the coveragediagram of the individual elements of the array. A broadside array isone in which the resultant beam pattern is normal to the axis of thearray and a squint-free broadside array is one in which this normalcondition is preserved over an appreciable range of frequencies. For ageneral description of the properties and characteristics of lineararray antennae reference may be had to chapter 9, volume 12, of theRadiation Laboratory Series, Massachusetts Institute of Technology.

The preferred embodiment of the invention to be disclosed in thisspecification is a broadside linear array when used as a beacon antenna.Employed in this capacity, the antenna is used as part of aninterrogation system working in conjunction with other primary search orallied radars. In this role, the transmitter associated with theinterrogator system sends out a pulse on one frequency and receives areply from the interrogated ob- 'ject, such as a ship, plane, tank,etc., on a different frequency. Since the system must transmit to, andreceive from, the same object, although on different frequencies, thedirectivity of the antenna must be preserved over a band of frequencies,that is, no squint must be introduced by the difference in the twofrequencies. Other types of antenna have been employed in this role butthe linear array, or more particularly the broadside array, are to bepreferred because of their peculiar property of being able to produce abeam of the required pattern from an antenna having an appreciablelength but relatively small width and depth. This enables it to bemounted on the same support structure as the antenna of the primaryradar, with little addition of weight and slight structural alteration.

The primary requirement of a broadside array when used in conjunctionwith a beacon system is that the array must retain its radiationproperties over an appreciable band of frequencies, in general a band20% about a given central frequency is considered desirable. The chiefproperties'which must be preserved over this frequency band are:

(a) The antenna azimuth pattern must change as little as possible.

(b) The vertical coverage must be quite broad and in general match thatof the associated primary radar.

(c) The beam width of the antenna must be the minimum possible,compatible with the maximum length of the array, which in turn isdictated by the size and configuration of the associated primary radar.

(d) Any secondary or side lobes should be as low as possible withrespect to the main beam, and must be down sufiiciently to permit theunambiguous transmission and reception of information.

(e) The impedance match of the antenna over the frequency range must bepreserved so that the voltage standing wave ratio (VSWR) is held withinreasonable limits.

The above conditions having been met, it is further required that themechanical properties of the antenna be given consideration, with a viewto reducing, as far as is practicable, the size and weight of theantenna, and also to render it as economically and simple produced aspossible.

Broadside arrays acting as beacon antennae and meeting to some extentthe above requirements, are known, however, they all suffer from one ormore deficiencies, the chief of which are:

(a) The power feed systems used to distribute power to the radiatingelements in the correct proportions to give the required beam shape withlow order side lobes, are all somewhat complex and required factoryfacilities not norm-ally available for field maintenance.

(b) The essential squint-free condition has not been met; that is, thebeam pattern changes its principal direction as the frequency isaltered. This is caused chiefly by a relative phase change in themicrowave energy emitted by the individual radiating elements.

(0) It has not proved practical to maintain, within acceptable limitsacross the required frequency band, the impedance match of the antennato the microwave transmitter and receiver. The principal impedancemismatch being introduced by the frequency selectivity of the individualradiating elements used.

A significant improvement in the above undesirable conditions is broughtabout in the antenna disclosed in this specification.

An object of this invention is to provide a radiating element for use inbroadside array antennae whose impedance is held within acceptablelimits over a prescribed frequency range.

Another object of this invention is to provide abroadside array antennawith a tapered power feed system which is simple and easy to make, andwhich can be readily maintained in field service.

A further object of the invention is to provide a broadside arrayantenna with a simple feeder system whereby the relative phase of themicrowave energy emitted by the individual radiating elements in thebroadside array is held constant over a broad frequency band.

The principles of the invention will be more readily understood from thefollowing etailed description of the preferred embodiment, inconjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of the antenna,

FIG. 2 is a plan view of the top of the antenna,

FIG. 3 is a cross sectional view of one of the radiating elements alongthe line III-III of FIG. 2,

FIG. 4 is a front elevation of one of the radiating elements with thebeam forming plates and power feed omitted,

FIG. 5 is a top plan view of one of the power feed sections with theupper conducting plate removed,

FIG. 6 is a perspective view of one of the power feed sections with theupper conducting plate shown in a raised position,

FIG. 7 shows diagrammatically the power distribution in a feed section,and

FIG. 8 shows diagrammatically the impedance network of a power feedsection.

With reference to FIGS. 1 and 2 it will be seen that the antennacomprises a linear series of radiating elements 3, ten being shown forpurposes of illustration though obviously the number used is dependentupon the performance characteristics required within the overridinglimitation of the physical space available. Energy which is fed intoeach of these radiating elements is radiated through two horizontalslots 4 in each element, the

3 resulting horizontal beam pattern being a composite func tion of thenumber of slots and the power distribution along them and the verticalcoverage being primarily controlled by the beam forming plates 1 whichextend the full length of the array.

Power is fed to each of the radiating elements through two identical,dissymmetrical, feed sections and 6 which are coupled by cables 7 toeach of the radiating elements and also to a T-junction 9, into which isfed the microwave energy from the associated transmitter.

Turning now to FIGS. 3 and 4 it will be seen that the radiating elementis essentially a length of waveguide formed into a boxlike structure byplates which short circuit each end, and having two identical horizontalslots 4 symmetrically displaced on either side of the vertical centreline of the broad face of the waveguide section but displaced from thehorizontal centre line through that face by approximately two thirds ofthe distance from this centre line to the top edge of the waveguidesection.

The power coming from the feed section 6 through the cables 7 enters theradiating element through a connector 8 and is introduced into thewaveguide section by a probe 10. The slots 4, due to their position onthe broad face of the waveguide section, transfer this energy into freespace.

The power feed sections 5 and 6 are, as stated, identical butdissymmetrical and the following description of power section 5 willtherefore apply with equal validity to power section 6.

The power feed section is required to accept microwave energy and todistribute this energy between the five radiating elements associatedwith this feed section in accordance with a definite ratio determined bymathematical analysis. This power division is done by a series ofquarter wave transformers where the impedance transformations areaccomplished by varying the diameter of a centre conductor positionedmidway between two conducting plates, the combination thus acting as anunbalanced line of varying impedance. 7

Referring to FIGS. 5 and 6 the two parallel conducting plates 11 and 13are separated by polystyrene spacers 12 which also serve to support thecentre conductor structure and provide the mounting for the outputconnectors 21. Power entering the feed section through the righthandconnector 21 is transmitted by the quarter wave section having centreconductor 14 to the first junction where it is divided between thequarter wave section having centre conductor 15 and conductor 23 inaccordance with the required impedance matching, the power fed to thecentre conductor 23 goes through connector 21 and then to the firstradiating element to the left of the centre line of the antenna. Thepower donated to quarter wave section 15 at this first junction is inturn further divided between quarter wave section 16: and the nextconductor 23 feeding the second element from the centre line on theantenna. This process of impedance transformation and coupling to one ofthe radiating elements is continued through quarter wave sections 16,17, 18, 19 and 20, section 20 being directly coupled to the connector 21for the last outer radiating element. It will be noted that certain ofthe impedance transformations are accomplished by varying the diameterof the center conductor in two steps i.e. two quarter wave sectionsrather than one, This, as will be described later, is due to thenecessity of keeping the impedance transformation in any one sectionbelow a certain limit.

As previously stated, the feed section is connected to the individualradiating elements by lengths of cable 7 having a common impedance.These cables are of varying length decreasing from the cables connectingthe first power take-ofis to the first radiating elements on either sideof the centre line of the antenna, down to the shortest cables whichjoin the ends of the feed sections to the 4 outermost radiatingelements. These cables are of such a length that the microwave energyreaching each radiating element is in phase with that reaching the otherelements, that is to say, the electrical length of the path from the Tsection 9 to each of the radiating elements 3 is the same.

In understanding the theory of operation of the antenna it is necessaryonly to consider one half of the antenna since the structure issymmetrical about the normal to the array and the following descriptionis, therefore, related to only one feed section feeding, in theembodiment described, five radiating elements, though of course thecomplete antenna would comprise two feed sections feeding ten radiatingelements.

Since it is felt that a better understanding of the invention willresult thereby, practical results are included in thefollowingdescription of the power feed section. This data is for a broadsidearray having ten radiating elements i.e. five elements associated witheach feed section, and a theoretical horizontal beam pattern in whichthe side lobes are 34 db down from the main beam. These figures are forillustration purposes only and should not be construed as placing anyrestrictions on the scope or spirit of the invention.

As stated, the correct power taper is determined by mathematicalanalysis. This is done by obtaining a formula from known radiationtheory for the power radiated by the half antenna in any directionbetween the normal to the array and its axis, substituting arbitraryconstants for the value of the current fed to each of the individualradiating elements, placing this formula in a form suitable forcomparison with a polynomial defining the curve which describes thetheoretical beam pattern required and, by equating coefiicients,obtaining the correct current ratios required at the radiating elementso as to produce this pattern. This method of comparing a generalizedradiation pattern formula with the idealized pattern given by the(Tchebysheff) polynomial appropriate to the number of radiating elementsused is common in the study of linear arrays, and a more detaileddescription will be found in any of the standard works of reference,including volume 12 of the M.I.T. Radiation Laboratory Series. Itsinclusion here is therefore unnecessary and it will suifice to list thecurrent distribution and hence the corresponding power distributiondetermined from a consideration of the general radiation formula and theassociated polynomial for the five elements of the half array underconsideration, as follows:

where 1;, I I I and I denote the calculated currents fed to the fiveelements of the half array, I being that for the innermost'element, Ifor the next, and so on to I for the outer element of the half array.The power taper required to give the correct azimuth beam pattern thusbeing known, the feed section is designed to divide the power itreceives from the transmitter in accordance with the ratios worked outfor the different elements. Its method of doing this uses three basicprinciples.

The first is that power fed to a Y-junction will divide between the twoarms of the junction in inverse proportion to the impedances presentedby the two arms. Considering a junction A having arms A and A power Pfed to this junction will be divided between the two arms A and A in theratio of 1 2:552 A2 AI where Z and Z are the impedances presented to theY-junction by the two arms of the junction.

The second principle concerns impedance transformation by quarter wavetransformers. A quarter wave transformer is a section of transmissionline or waveguide having a length equal to one quarter of the wavelength of the frequency to be propagated and it can be shown that forsuch a quarter wave length of transmission line terminated in animpedance Z,, the impedance looking into the quarter wave length soterminated is given by where Z is the characteristic impedance of thequarter wave length line and Z, is the impedance looking into this line.Conversely where the required terminating and input impedances are knownit will be seen that the characteristic impedance of the quarter wavelength of line required to bring about this transformation is given by Z/Z,Z,

Where the impedance transformation is a large one it is found that thecharacteristic impedance of the line would reach an undesirably highvalue if only one quarter wave section were used. This is due to thefact that, as will be seen from the next section, the higher theimpedance the narrower the centre conductor in a parallel plate systembecomes. This makes for difficult manufacture and lack of mechanicalrigidity. In addition an unusually high impedance transformation resultsin a more rapid deterioration in impedance match as the frequency ischanged. For these reasons two quarter wave sections are used whereverthe characteristic impedance required would be high using only onequarter wave section. In this case, denoting the characteristicimpedances of the two quarter Wave sections by Z and 2 respectively,then 2,, the input impedance looking into the two quarter wave sectionsterminated by an impedance Z is given by The third basic principlegoverns the means of obtaining the characteristic impedance required.This is done by controlling the relative dimension of a centre conductorin between two parailel conducting plates when the characteristicimpedance is given by where Z is the characteristic impedance of theline, K is the dielectric constant of the material used, which could beany non-conducting medium, such as polystyrene, though in this casesince simplicity and ease of manufacture were desired, air was used,

It is the distance between the two parallel conducting :plates, and

d is the size of the centre conductor.

Thus, for a fixed value of h the characteristic impedance may be raisedby reducing d, that is reducing the size of the centre conductor.However, for high values of characteristic impedance, the centreconductor may become so small that it becomes undesirable from amechanical point of view, in which case, as previously stated, theimpedance transformation is made in two steps, using quarter wavesections with lower characteristic impedances.

In FIG. 7 is shown a diagrammatic representation of the power enteringand leaving the feed section. It will be seen that it consists of aseries of Y-junctions A, B, C, D, at each of which power entering thejunction is fed into two arms, one arm at every junction being connectedto one of the radiating elements, the other arm transmitting power forthe remaining elements to be fed. Thus power P entering the feed systemat E is divided at junction D into power P which proceeds on to junctionC, and power P feeding the first radiating element in the ratio where Zand Z are the impedances presented to the junction D by its two arms.

Power P is, in turn, further divided into P and P controlled byimpedances Z and Z presented by its arms to junction C, and so on downthe line to the last junction A where the power is divided between there- .maining two radiating elements. The known factors here ,are PAI,PAQ, P32, peg and P132 have been determined from the previouslymentioned mathematical analysis. Since P and P are known, then is alsoknown and hence P can be found. Also known are the values of theimpedances of thearms of the various junctions connecting directly tothe radiating elements, namely Z Z Z and Z These are all identical andare of known value, namely the characteristic impedance of the matchedcable and radiating element.

Considering then firstly junction A, P and P are known and also Z Thussince the required power division is known the required impedancedivision is known and 2 can be found.

Considering junction B, P is known, Z is known and P can be found byadding P and P and hence Z can be found, and so on for the remainingjunctions C and D. It is thus possible to determine the power ratiorequired between the two arms at each of the junctions and hence the twoimpedances required to accomplish this power division. The followingtable gives these values for the adduced example of a five elementarray:

TABLE Junction A B C D Power in Arm 1 PA1=L00 PB1=PA=4-98 PCi=PB= 6-O0P131=Pc=36.51

Power in Arm 2 PAr=3.98 Pni=1l.02 PC2=20.51 P =27.59

Power entering junction PA=PA1 PB=P Bl PBZ PC=PCi Poz PD=PD1 Pm Ratio:

Power in Arm 2 P 12 11.02 2 20.51 13 27.59

Power in Arm 1 1251.00 Pm 4.9a Pcr 16.00 Pm 3fi.5l

Which equals:

Impedance of Ann 1 ZAI ZB1 c1 Z D 1 Impedance of Ann 2 Zn zrf Zcf ZmSince Z =Z =Z =Z =Z (50 ohms) the common impedance of a radiatingelement and its associated feed cable, Z Z Z and Z can be found The nextrequirement is that each of the junctions has the correct impedancespresented to it. Referring to FIG. 8, the required value for Z theimpedance presented by its arm 1 to junction A, has been calculated, butthis does not correspond to the terminating impedance of arm A which isZ the common impedance of a radiating element. It is therefore necessaryto transform impedance Z to Z This is done by two quarter wavetransformers having characteristic impedances Z and Z calculated in themanner previously described using the known values of terminating andinput impedances required. Looking next into junctionA from junction B,junction A has an impedance Z given by l/Z is equal to 1/Z +l/Z Since Zand 2 are known, Z A can be calculated. Again it does not, however,equal the required impedance Z which must be presented at the junction Bto ensure the correct power division at this junction. Impedance Z musttherefore be transformed by two quarter wave sections Z0131 and Z toimpedance Z This process is continued for each of the junctions with theoutput impedance of junction B, namely Z beingtransformed to Z Z to Zetc. a final transformation being on the output impedance of junction D,namely Z which has to be transformed by the quarter wave section havingcharacteristic impedance Z into impedance Z rwhich is the characteristicimpedance of the feeder cable feeding the power feed section.

The following table gives the impedance transformations required and thecharacteristic impedance-s necessary to accomplish them for the examplecited:

radiatingelement concerned is made aqual by adjusting the length of theconnecting cable to each element, though in certain circumstances itcould prove of value to have the electrical paths of unequal length,such as when an unequal inter-element phase relationship is required.Hence the microwave energy supplied to all radiating elements'has acommon phase and this uniphase condition is preserved at allfrequencies.

The remaining conditions required of the array are, that the radiatingelement accept all of the power denoted to its by the feed section andtransmit this power into free space through the horizontal slots cut init; that the radiating element presents to the connecting, cable thecorrect impedance match for this cable, and that each radiating elementis not affected by adjacent radiating elements, since this would disruptthe power distribution and hence the azimuth beam pattern.

The basic theory associated with the element is that of a slot cut in awall of a section of waveguide parallel to, but displaced from, thecentre line of the wall. Under these condition the slot acts as aradiator and if displaced a suitable distance from the centre line willprovide unity normalized conductance looking along the waveguide towardsthe slot. When this condition is achieved, substantially all the powerin the waveguide is radiated by the slot with-out reflection. Where morethan one slot is used, then the conductance must be adjusted so that foreach element its normalized value is inversely proportional to thenumber of elements used, that is l/n where n is the number of slots. Theradiating elements of the array have two such slots and these are spacedone half guide wavelength apart so that they are effectively in paralleland the waveguide section is short circuited one quarter guidewavelength outwardly beyond each slot so that the admittance in parallelwith each slot is zero. It may at first sight appear that each elementshould have only one slot and this could be done, but, by symmetricallyplacing the probe feed it is possible to employ two slots and stillTABLE Section A 11-13 13-0 0-1) 13-13 ZAIZA2 Bi Bz oi oi ZmZm u ZTerminating Impedance 1:. 50 A ZA1+ZA2 B 8 +Zm v c ZCI+ZC2 D zm+zm InputImpedance ZA1=199 Zn1=110.5 Zci=64 Zm=38 ZE=50 Characteristic Impedance:

Section 1 Zon1= v zm zo Zom= 'V BI ZA Z0c1= 'VZClZB Zor 1= VZmZc ZOE1=VZDZE 1199 .50 V110.5 .4O V64.34.4 13828.1 V21.6.50

Section 2 Zonz= w ZmZo Zonz= /Zn1ZA The quarter wave transformers usedin the [feed section are, of course, to some degree frequency selectiveand provide their optimum transformation only at the design frequency.Deviations from this frequency introduce an impedance mismatch with acorresponding reflection of some power and some disruption of thedistribution of power to the associated radiating elements. This effectis negligible however for slight deviations and does not cause a seriousdeterioration in the performance of the feed section within a frequencyband 10% on either side of the design frequency.

Thus the power feed section supplies the correct power to each of itsassociated radiating elements and, as previously stated, the electricallength of the path taken by this power \from the input to the feedsection to the individual preserve the in phase condition; this has thedecided advantage of halving the number of feed points required. Sincethe two slots are symmetrically placed they are thus in parallel withthe feed probe and must therefore be so displaced from the centre lineas to present a normalized conductance of /2 to the probe. The radiatingelement is therefore a section of waveguide, one guide wavelength longwith two horizontal slots one half wavelength apart and each one quarterwavelength from the short circuit plates at the end of the waveguidesection and both displaced upon the centre line of the waveguide such adistance that the two slots in parallel present unity normalizedconductance. The slots themselves are quite wide so as to preserve theirconductance match over an appreciable frequency range, and

since, as is general, the perimeter of each slot is made one guidewavelength, the length of each slot is somewhat less than one half ofthe guide wavelength.

The probe which couples the energy into the waveguide setcion is a pieceof tubing having a broad band characteristic and 'is of such a lengththat it matches the normalized conductance of the two slots seen inparallel from the probe to the feeder cable. This length thoughcalculated approximately is finally determined by experiment.

Of the three conditions required of the radiating elements therefore,the unity conductance factor is preserved over an appreciable range offrequencies and the voltage standing wave ratio of a radiating elementwhich represents the amount of power reflected from the element wasfound to be very good over the 20% frequency band required in theexample given above. The impedance matching condition is determined bythe probe and due to the broad band characteristics of the slot and theprobe it is also held within acceptable limits over the 20% band.

The requirement that minimum mutual impedance shall exist between slotsis one of the characteristics of this type of radiator since between aslot end to end with another parallel slot there i little or no mutualimpedance.

The anntenna is therefore a broadside linear array for use at microwavefrequencies, and is made up of a series of radiating elements disposedalong a linear axis. Each of these radiating elements is a section ofwaveguide, resonant at a given design frequency, and short circuited ateach end. Power is radiated from the element by slots, parallel to thelinear axis of the array, acting as radiators, cut into that wall (orwalls, since, if desire-d, the antenna could radiate in more than onedirection) of the section which is contiguous with those like walls ofthe other elements which together form the radiating face, or faces, ofthe array. Though the element would radiate if only one slot were usedin a radiating wall of the section, it is desirable to have two slots,end to end and working in conjunction, in any radiating wall since thisreduces the number of feed points. The slots (or slot) are displaced[from the centre line of the wall parallel to their linear axis by sucha distance that their normalized conductance viewed along the waveguideis unity, which is the condition required to give maximum transfer ofenergy from the waveguide section to free space. The slots are spacedone half design frequency guide wavelength apart and the waveguide shortcircuits are placed one quarter guide wavelength outwardly beyond eachslot, so that the slots are in parallel, and the admittance in parallelwith the slots due to the short circuit is zero.

The waveguide section is excited by a probe placed symmetrically withrespect to the slots, in one face of the section; this probe is designedto provide a correct match from the radiating element to the feedercable supplying power to it.

Power is conveyed to each of the radiating elements by a system offeeder cables whose lengths are adjusted to control the phase of theenergy reaching the element associated with each cable, in general thisphase being the same for all elements.

These cables connect the radiating elements to a power distributionsystem which includes one or more feed sections, whose function is toaccept power from a microwave transmitter and distribute this poweramongst the radiating elements associated with the section in apredetermined proportion so that each element gets that power necessaryto give the array its desired azimuth beam pattern. This is done in thefeed section by a series of quarter wave transformers, whose impedancetransformation control the power division at a series of Y-junctions,the characteristic impedance of each transformer section being governedby the relative dimension of a centre conductor, between two parallelplates -in a dielectric medium, which together with the centre conductorconstitute an unbalanced transmission line.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A broadside linear array antenna for use at microwave frequenciescomprising, in combination, a series of radiating elements disposedalong a linear axis, each of said radiating elements being a section ofwaveguide short circuited at each end, resonant at the prescribed designfrequency, which has, on at least one one of its walls, which with thelike wall on adjacent radiating elements forms a radiating face of thearray, at least one radiator slot parallel to the linear axis of thearray and placed from the axial centre line of said wall by a distancesuch that the normalized conductance of the total number of slots isunity; an impedance matched power distribution system for supplyingmicrowave energy to said radiating elements including at least one powerfeed section capable of accepting microwave energy from a source anddistributing this microwave energy by consecutive impedancetransformations in predetermined proportions amongst a series of powertake offs connecting to the radiating elements associated with the feedsection, said impedance transformations being accomplished by a seriesof quarter wave transformers whose characteristic impedance iscontrolled by the relative dimension of a centre conductor between twoparallel conducting plates in a dielectric medium; and an appropriatenumber of feeder cables connecting said power take-off to saidassociated radiating elements, the length of each said feeder cablesbeing adjusted to control the relative phase of the microwave energyreaching said radiating elements.

2. A broadside linear array antenna for use at microwave frequencies asdefined in claim 1 wherein the quarter wave transformers of said powerfeed section have common parallel plates a fixed distance apart, andobtain their required characteristic impedance by the suitabledimensioning of the diameter of a round centre conductor fixed midwaybetween the two parallel plates, and aid dielectric medium is air.

3. A broadside linear array antenna for use at microwave frequencies asdefined in claim 1, wherein said radiating element has two radiatorslots in said wall of said waveguide section displaced from the axialcentre line of said wall by a distance such that the normallizedconductance of each slot is /2.

4. A broadside linear array antenna for use at microwave frequencies asdefined in claim 3 wherein said radiator slots are spaced one half of aguide wavelength apart and the waveguide section is short circuited onequarter guide wavelength outwardly beyond either slot.

References Cited by the Examiner UNITED STATES PATENTS 2,602,856 7/1952Rumsey 333--35 X FOREIGN PATENTS 905,384 3/1954 Germany.

ELI LIEBERMAN, Primary Examiner.

CHESTER L. JUSTUS, Examiner.

R. E. BERGER, Assistant Examiner.

1. A BROADSIDE LINEAR ARRAY ANTENNA FOR USE AT MICROWAVE FREQUENCIESCOMPRISING, IN COMBINATION, A SERIES OF RADIATING ELEMENTS DISPOSEDALONG A LINEAR AXIS, EACH OF SAID RADIATING ELEMENTS BEING A SECTION OFWAVEGUIDE SHORT CIRCUITED AT EACH END, RESONANT AT THE PRESCRIBED DESIGNFREQUENCY, WHICH HAS, ON AT LEAST ONE ONE OF ITS WALLS, WHICH WITH THELIKE WALL ON ADJACENT RADIATING ELEMENTS FORMS A RADIATING FACE OF THEARRAY, AT LEAST ONE RADIATOR SLOT PARALLEL TO THE LINEAR AXIS OF THEARRAY AND PLACED FROM THE AXIAL CENTRE LINE OF SAID WALL BY A DISTANCESUCH THAT THE NORMALIZED CONDUCTANCE OF THE TOTAL NUMBER OF SLOTS ISUNITY; AN IMPEDANCE MATCHED POWER DISTRIBUTION SYSTEM FOR SUPPLYINGMICROWAVE ENERGY TO SAID RADIATING ELEMENTS INCLUDING AT LEAST ONE POWERFEED SECTION CAPABLE OF ACCEPTING MICROWAVE ENERGY FROM A SOURCE ANDDISTRIBUTING THIS MICROWAVE ENERGY BY CONSECUTIVE IMPEDANCETRANSFORMATIONS IN PREDETERMINED PROPORTIONS AMONGST A SERIES OF POWERTAKE OFFS CONNECTING TO THE RADIATING ELEMENTS ASSOCIATED WITH THE FEEDSECTION, SAID IMPEDANCE TRANSFORMATIONS BEING ACCOMPLISHED BY A SERIESOF QUARTER WAVE TRANSFORMERS WHOSE CHARACTERISTIC IMPEDANCE ISCONTROLLED BY THE RELATIVE DIMENSION OF A CENTRE CONDUCTOR BETWEEN TWOPARALLEL CONDUCTING PLATES IN A DIELECTRIC MEDIUM; AND AN APPROPRIATENUMBER OF FEEDER CABLES CONNECTING SAID POWER TAKE-OFFS TO SAIDASSOCIATED RADIATING ELEMENTS, THE LENGTH OF EACH SAID FEEDER CABLESBEING ADJUSTED TO CONTROL THE RELATIVE PHASE OF THE MICROWAVE ENERGYREACHING SAID RADIATING ELEMENTS.